Waxy feedstocks may be used to prepare basestocks having a high viscosity index (VI). However, in order to obtain a basestock having the low temperature properties suitable for most uses, it is usually necessary to dewax the feedstock. Dewaxing may be accomplished by means of a solvent or catalytically. Solvent dewaxing is a physical process whereby waxes are removed by contacting with a solvent, such as methyl ethyl ketone, followed by chilling to crystallize the wax and filtration to remove the wax. Catalytic dewaxing involves chemically converting the hydrocarbons leading to unfavorable low temperature properties to hydrocarbons having more favorable low temperature properties. Long chain normal paraffins and slightly branched paraffins readily solidify and thus result in generally unfavorable low temperature properties. Catalytic dewaxing is a process for converting these long chain normal paraffins and slightly branched paraffins to molecules having improved low temperature properties.
Catalytic dewaxing may be accomplished using catalysts that function primarily by cracking waxes to lower boiling products, or by catalysts that primarily isomerize waxes to more highly branched products. Catalysts that dewax by cracking decrease the yield of lubricating oils while increasing the yield of lower boiling distillates. Catalysts that isomerize do not normally result in significant boiling point conversion. Catalysts that dewax primarily by cracking are exemplified by the zeolites ZSM-5, ZSM-11, ZSM-12, beta and offretite. Catalysts that dewax primarily by isomerization are exemplified by the zeolites ZSM-22, ZSM-23, SSZ-32, ZSM-35, ZSM-48 and ZSM-50. To ensure adequate mechanical strength for use in a dewaxing reactor, such zeolite catalysts are generally combined with an inorganic oxide binder, such as alumina.
Catalysts are needed for the upgrading of renewable basestocks for fuels and lubricant applications. For example, a catalyst for fatty acid coupling helps production of a highly flexible feedstock. As shown in FIG. 1, this feedstock can then be hydrogenated and/or isomerized using conventional refinery processing, thereby producing high value products consisting of a mixture of fuels, high viscosity, and low viscosity lubricants. This product stream can easily be separated using conventional fractionation and distillation equipment.
The hydrogenation/isomerization catalyst for renewable feedstocks has several challenges to deal with: 1) a highly oxygenated feed (10% oxygen), 2) high heats of reaction, and 3) generation of water which is converted into steam in the reactor. The last challenge is of major concern to current dewaxing catalysts because steam can cause issues with the hydrothermal stability of the catalyst and can cause deactivation by dealuminating the zeolite catalyst and/or degradation of the oxide support/binder leading to agglomeration of the metal.
Conventional dewaxing catalysts are, however, susceptible to poisoning by contaminants in a feedstock. To mitigate the problem of catalyst poisoning and to allow effective dewaxing of feedstocks with very high levels of waxy materials, it is often desirable to be able to maximize the dewaxing activity of the catalyst. However, in seeking maximize activity, it is also important to maintain the mechanical strength of the catalyst.
U.S. Pat. No. 8,263,517 to Christine N. Elia describes a dewaxing catalyst comprising a zeolite with a low silica to alumina ratio in combination with a low surface area binder. The low surface area binder is believed to increase access to the active sites of the zeolite. Especially for bulky feeds, increased access to zeolite active sites is expected to lead to an overall increase in activity.
U.S. Patent Publication No. 2011/0192766 mentions a supported catalyst comprising a zeolite having a silica to alumina molar ratio of 500 or less, a first metal oxide binder having a crystallite size greater than 200 Å and a second metal oxide binder having a crystallite size less than 100 Å, wherein the second metal oxide binder is present in an amount less than 15 wt % of the total weight of the catalyst.